For one set, the camera pointed nearly straight up. For the other, it pointed just above the southern horizon.

Cloud movement was recorded in both and was made easier to see by image enhancement. A midday look at the sky with the same camera the same day showed no clouds, NASA said.

"It is likely that the clouds are composed of crystals of water ice that condense out onto dust grains where it is cold in the atmosphere," said Curiosity science-team member John Moores of York University in Canada.

"The wisps are created as those crystals fall and evaporate in patterns known as 'fall streaks' or 'mare's tails.' While the rover does not have a way to ascertain the altitude of these clouds, on Earth such clouds form at high altitude," said Mr Moores.

Mars' elliptical orbit makes that planet's distance from the Sun vary more than Earth's does.

In previous martian years, a belt of clouds has appeared near the equator around the time Mars was at its farthest from the Sun, according to the US space agency.

The new images of clouds were taken about two months before that farthest point in the orbit, relatively early in the season for the appearance of this cloud belt.